1. Field of the Invention
The present invention relates to a process for vapor-phase polymerization of monomers in a vapor-phase, quench-cooled reactor vessel in which off-gas from the reactor vessel is recycled. More specifically, this invention relates to a method and apparatus for minimizing the formation of polymer agglomerates or lumps in a reactor used in the gas-phase polymerization of polypropylene by controlling the location of injection of a catalyst into the reactor on one hand and a cocatalyst and modifier into the reactor on the other hand while maintaining catalyst yield and stereospecificity as well as polymer physical properties.
2. Description of the Prior Art
Vapor-phase polymerization processes using vapor-phase, quench-cooled reactor vessels have been described in a number of U.S. patents including: U.S. Pat. Nos. 3,957,448, 3,965,083, 3,971,768, 4,101,289 and 4,372,919, all incorporated by reference herein. These references describe polymerization processes and apparatus in which polymer is formed from gaseous monomer in horizontal stirred bed vessels.
Typically, in operation of such processes and apparatus, particles of polymer are formed around solid catalyst particles.
The horizontally disposed reactor vessel typically has recycled propylene gas introduced into the bottom thereof together with hydrogen gas. Then, liquid propylene, as a quench liquid, is injected into the reactor from the top of the reactor. The hydrogen is provided for molecular weight control.
Paddlewheels or other types of agitators inside the vessel stir the contents of the vessel.
At a so-called upstream end of the vessel a catalyst system comprising a catalyst injected at one point into the top of the vessel, and a cocatalyst plus modifier injected at a point adjacent the point of injection of the catalyst, is injected into the top of the vessel.
Solid polypropylene powder is created in the vessel and is withdrawn from the downstream end thereof.
It is desirable to create polymer particles as quickly as possible, and for this purpose a number of different high activity catalyst systems have been developed.
However, one of the problems that has been incurred with such vapor-phase polymerization processes and apparatus using a vapor-phase, quench-cooled horizontally disposed reactor vessel is the formation of lumps in the powder polymer products within the reactor vessel and in the powder polymer product.
Lumps in the product result in a nonuniform size product and lumps inside the reactor vessel can result in stoppage of the process requiring cleaning of the reactor vessel before the process can be continued. This can be quite costly and time consuming.
As will be described in greater detail hereinafter, the method and apparatus of the present invention minimize the formation of polymer agglomerates or lumps. Such minimization of polymer agglomerates or lumps has been brought about by the discovery that if the catalyst is introduced into the reactor vessel adjacent to the upstream end of the reactor vessel and then the cocatalyst plus modifier are introduced into the reactor vessel at a sufficient distance downstream from the point of introduction of the catalyst, formation of polymer agglomerates or lumps is minimized, if not altogether avoided.
Although not known with absolute certainty, it is believed that separating the introduction of the catalyst and cocatalyst plus modifier facilitates the gradual activation of the catalyst by thorough mixing of the catalyst with the polymer material within the reactor vessel prior to the interaction of a high concentration of the cocatalyst plus modifier with the polymer material and catalyst resulting in a reduction, if not altogether avoidance, of the formation of polymer lumps within the reactor vessel.